Memory devices typically include an array of memory cells arranged in rows and columns. The memory cells in each of the rows are normally activated by applying an activation signal to an access line, commonly referred to in the art as a word line, for their respective row. The activation signal is normally generated by a row address decoder, which decodes addresses received by the memory device.
A typical prior art row address decoder 10 and word line driver 20 is shown in FIG. 1. The row address decoder 10 includes four NMOS transistors 12-18 having their drains and sources connected in series with each other. The gates of the transistors 12-18 each receive a respective bit of a row address or another signal corresponding to a row address bit. The source of the first transistor 12 also receives and decodes a bit. More specifically, the source of the transistor 12 receives an LsecF signal, which is an active low bit corresponding to a section of a memory array containing a plurality of rows. The gate of the transistor 12 receives a global phase signal GPH corresponding to the most significant bit of the row addresses in the respective section. The gate of the next transistor 14 receives a signal RB, which corresponds to the next to least significant bit of a row address. Finally, the gate of the transistor 16 receives a signal RA, which corresponds to the least significant bit of a row address. The remaining transistor 18 in the row address decoder 10 is connected to a supply voltage VCC, which maintains the transistor 18 in a conductive state. The function of the transistor 18 will be explained below.
In operation, a specific row corresponding to the row address decoder 10 is decoded when the section signal LsecF for that row is active low, and the global phase signal and the row bits GPH, RB, RA, respectively, for that row are active high. In such case, the row address decoder 10 outputs an active low signal Pc. In all other cases, the output of the row address decoder 10 is in a high state. Although the row address decoder 10 shown in FIG. 1 decodes only a section bit, a global phase signal, and two row address bits, it will be understood that row address decoders having similar typography are in use to decode a fewer or greater number of row address bits and other signals corresponding to or derived from address bits.
The word line driver 20 performs the function of generating a high word line signal WL responsive to receiving an active low signal Pc from the decoder 10. The word line driver 20 includes a PMOS transistor 22 receiving an active low precharge signal GPcF, and a latch 24 formed by a pair of cross-coupled PMOS transistors 26, 28. The PMOS transistor 28, in combination with an NMOS transistor 30, forms an output stage that drives the word line WL. All of the transistors except for the NMOS transistor 30, have their sources connected to a precharge supply voltage VCCpr, which is a pumped voltage above the supply voltage VCC.
In operation, the latch 24 is reset at the end of a row access by the transistor 22 receiving an active low precharge signal GPcF, which turns off the PMOS transistor 28, and turns ON the NMOS transistor 30 to drive the word line WL low. The low word line voltage turns ON the PMOS transistor 26 to maintain the voltage applied to the gate of the NMOS transistor 30 high.
When the row address decoder 10 decodes a row address for the respective row, the decoder 10 outputs a low Pc signal. This low Pc signal sets the latch 24 by pulling the gate of the PMOS transistor 28 low to turn ON the transistor 28 and drive the word line WL high. The low Pc signal also turns OFF the NMOS transistor 30. At the same time, the high word line voltage turns OFF the PMOS transistor 26. The high voltage of the WL then activates the memory cells (not shown) in the row to which the word line WL is connected. As explained above, at the end of the access, the precharge signal GPcF is driven active low to reset the latch 24 and place the word line driver 20 in the original state.
As indicated above, the row address decoder 10 includes an NMOS transistor 18 that is turned ON by the supply voltage VCC being applied to its gate. The function of the transistor 18 is to reduce the effects of channel hot carriers “CHC” on the transistors 12-16. The CHC phenomena occurs when current begins to flow through a transistor with a high drain-to-source voltage. In such case, the high drain-to-source voltage causes the electrons flowing through the transistor to accelerate to a high velocity. The high velocity of these electrons may cause them to be injected into the gate oxide of the transistor, thereby resulting in damage to the gate oxide. Insofar as a high drain-to-source voltage maximizes the CHC damage, the danger of CHC damage is generally at its worst as the transistor becomes conductive. If the drain of the transistor 16 was connected directly to the latch 24, then the drain of the transistor 16 would be at the precharge supply voltage VCCpr when the row address decoder 10 began to decode an address since the latch 24 would then be reset. In such case, the source of one or more of the transistors 12-16 would be low when the transistors 12-16 turn ON, thereby placing the full magnitude of the precharge supply voltage VCCpr across one or more of the transistors 12-16 as the transistors 12-16 are turned ON. As a result, the transistors 12-16 could be damaged by the CHC phenomena. The presence of the transistor 18 maintains the voltage on the drain of the transistor 16 at the supply voltage VCC less the threshold voltage VT of the transistor 18 when the latch 24 is reset. This reduced gate-to-source voltage of the transistor 16 avoids CHC damage in the transistors 12-16.
The prior art row decoder 10 and word line driver 20 shown in FIG. 1 has, in the past, provided acceptable performance. However, as memory densities increase, the row pitch, i.e., the spacing between rows, makes it more difficult to accommodate the CHC protection transistor 18. Furthermore, when the latch 24 is in a reset condition to hold the voltage of the word line WL low, the high drain-to-source voltage of the transistor 28 when it turns ON can cause CHC damage to the transistor 28. Similarly, when the latch is in a set condition, the voltage on the gate of the transistor 30 is low and the voltage of the word line WL is high. In such case, the gate-to-source voltage of the transistors 22, 30, 26 is equal to the precharge supply voltage VCCpr so that CHC damage to these transistors can occur when the output of the row address decoder 10 transitions low in response to decoding a row address.
Another limitation of the word line driver 20 shown in FIG. 1 is a relatively slow switching time and high power consumption of the transistors 28, 30. The word line WL is normally relatively long thereby causing it to have a substantial capacitance. As a result, it can require a considerable period for the transistor 28 to drive the word line WL from low-to-high. This delayed transition can limit the operating speed of a memory device. The delayed transition also maintains the PMOS transistor 26 ON for a considerable period so that it is unable to turn OFF the PMOS transistor 28. As a result, both the PMOS transistor 28 and the NMOS transistor 30 are on for a considerable period thus resulting in considerable current flow through these transistors 28, 30, which results in a high power consumption.
Still another limitation of the word line driver 20 may be excessive gate-induced drain leakage (GIDL). Gate-induced drain leakage results in current flowing between the drain and the substrate of a MOSFET transistor that is in a non-conductive state when the gate voltage of the transistor is too high. This GIDL current is the result of a high electric field developed in an area of the substrate where the gate overlaps the drain of the transistor. This GIDL current is undesirable for a variety of reasons. In the word line driver 20 of FIG. 1, the voltage on the gate of the PMOS transistor 28 is substantially equal to the precharge power supply voltage VCCpr when the latch 24 is in a reset state so that the word line WL is not being driven. This precharge power supply voltage VCCpr can be large enough to result in undesirable GIDL current in the transistor 28. For essentially the same reason, GIDL current can flow through the transistors 22, 26 when the latch 24 is in a set state.
There is therefore a need for a row decoder and/or word line driver that provides fast response time, avoids CHC damage to the transistors in the row decoder and/or word line driver and/or avoids generating GIDL currents in the transistors in the row decoder and/or word line driver.